The present invention relates to the purification of silicon.
Silicon that is used in the manufacture of solar cells must have a minimum purity that is referred to here as solar grade (SG) silicon. SG silicon has significantly higher purity than a lower metallurgical grade (MG) silicon, although solar grade can be lower than electronic grade (EG) silicon, which is used for manufacturing semiconductor devices. While MG silicon can have up to 10,000 ppm of impurities and EG silicon requires less than 1 ppb of donor or acceptor impurities, SG silicon should have no more than 5 ppm of metallic impurities.
To remove from a batch of silicon impurities that have low segregation coefficients, it is well known to provide directional solidification so that the impurities with low segregation coefficients can be segregated to the last part of the melt to solidify these impurities for removal. To remove impurities with high segregation coefficients, however, particularly boron and phosphorus, MG silicon is typically converted to a gaseous product and then purified by distillation.
A number of efforts have been made to efficiently produce SG silicon as an intermediate grade between MG silicon and EG silicon. In U.S. Pat. No. 5,182,091, for example, MG silicon is heated to a molten state in a refractory-lined crucible with a heating coil wrapped around it. A high-temperature, high-velocity plasma jet directs an inert gas with steam and/or silica powder from a height of 50 mm to produce a hot spot where boron and carbon escape. This approach requires the use of a plasma generator, which adds complexity and expense to a purification system.
In an article by Baba, et al, xe2x80x9cMetallurgical Purification for Production of Solar Grade Silicon from Metallic Grade Silicon,xe2x80x9d a rather costly four-step process is described for refining small quantities of MG silicon. With this process, phosphorous is removed with an electron beam gun and the silicon melt is directionally solidified. Then, boron and carbon are removed by blowing argon plasma with water vapor into the melted silicon, and a second directional solidification process is performed. This process requires an hour to remove phosphorus and boron from just a few kilograms of liquid silicon, and also requires use of a plasma generator.
Other efforts have been made to produce SG silicon with methods that would provide lower cost than that required to produce EG than silicon. Such efforts have involved using higher purity raw materials in an arc furnace, acid leaching, reactive gas treatments in a molten state, slagging, and dissolution of MG silicon in a metal followed by recrystalization. None of these processes, however, has effectively removed a sufficient amount of impurities, particularly boron and phosphorous, in a cost-effective manner.
It would be desirable to have an efficient method for purifying large amounts of MG silicon to produce SG silicon.
The present invention includes methods and apparatus for efficiently purifying MG silicon to produce SG silicon. In one aspect of the present invention, a system for purifying MG silicon includes a container for holding molten silicon, and a heater that can be immersed in the molten silicon. The immersion heater preferably includes an oxygen-hydrogen torch that has a flame surrounded by an inert gas, such as argon, so that the torch provides heat, water vapor, and the inert gas. The inert gas provides space for the flame, and also can be used to carry silica (SiO2) powder to the flame and generate turbulence within the molten silicon. The torch can have a flame surrounded by air. The immersion heater can be a gas lance surrounded by air and/or other combustible gases. In addition to silica powder, the torch or lance can carry water vapor or other reactive powder, liquid, or gas in addition to or instead of the silica powder. The heater can also be used above the melt and the distance above the melt and the flow can be controlled to cause turbulence and stirring.
The container can also include a system for directionally solidifying the melt, e.g., with a cooling system that includes a tank in which the container is held, an inlet for providing a coolant, a plurality of outlets at different vertical positions relative to the container, and a controller for controlling the physical vertical level of the coolant. Alternatively, the melt can be provided into another container for directional solidification, with or without a vacuum.
Accordingly, the method includes prolonging the reaction time for the purification while the silicon is in a molten state by using a torch or lance, and following this prolonged reaction time with directional solidification, with or without evacuation. The torch can be an immersion heater, or the torch can have passages (preferably concentric, although possibly side-by-side) for providing oxygen and hydrogen. Such a torch can be used to direct heat from above the melt with a flame, and without use of plasma or a plasma torch.
A torch is provided over the melt and provides oxygen, hydrogen, and other additives. These additives can include silica powder and CaO, BaO, or CaF2, which are generally known for use in a slag, as shown in U.S. Pat. No. 5,788,945. The use of CaF2 and other fluxes, however, can be undesirable because they can degrade the crucible that holds the silicon.
In another aspect of the present invention, alumina (Al2O3) is added to lower the melting point of the slag and is stable to still allow for removal of Al from the melt. In addition, other additives such as CaO, BaO are added to make the slag more basic. Basic slags have a higher capacity to capture and return impurities across outer slags. The alumina is used in a sufficient quantity as desired to further lower the melting point of the slag, and to allow the slag to tolerate changes in silica content and remain fully molten. The alumina thus avoids the need to use a flux. Further, the density of the slag can be controlled by adding oxide, for example BaO or CaO, and thus determine whether it is a floating slag or a sinking slag.
In another aspect of the present invention, a method includes steps of submerging an immersible heater, such as a torch, within molten silicon to heat the molten silicon, and preferably also to provide inert gas to permit combustion to generate heat and water vapor and to carry silica powder and to create turbulence to expose more silicon. The torches can be submerged near the bottom of the container, and as processing continues, are raised within the container to assist with directional solidification. A directional solidification step can include raising the torches, and also preferably includes controlling heat extraction from the container.
In another aspect, the invention includes a method of maintaining molten silicon in a liquid state and purifying the molten silicon by stirring, slagging, reaction with moisture, oxidation, evacuation, and reduction. This can be done with or without immersion heating. The torch can be over the melt and have a flame to provide heat. A heater can be provided in or around the crucible for holding the silicon, in which case the torch may not even be needed to provide a flame or heat, but can be used as a lance to introduce oxygen and hydrogen gas in separately controllable amounts.
The present invention provides an effective and efficient mechanism for purifying molten MG silicon to produce SG silicon, in a way that can be done on a large scale as the MG silicon is being produced. This benefit is accomplished without the need for creating a plasma jet. The method includes purifying the silicon with chemical reactions so that products are volatilized or entrapped in slags, enhancing the reaction rate by heating and stirring the melt and by controlling its composition, prolonging the reaction by providing a heat source to keep the silicon in the molten state longer, and controlling the solidification to enhance the purification by the effects of segregation. The torches or gas lances can provide one or more of heat, turbulence, water vapor, silica powder, an additive to make the slag more basic, alumina, and inert gas, all of which are or can be useful and/or necessary in the purification of molten silicon. In the case of torches or gas lances, the oxygen/hydrogen ratio can be controlled to optimize chemical reactions. Other features and advantages will become apparent from the following detailed description, drawings, and claims.